Planet
navi homePPSaboutscreenshotsdownloaddevelopmentforum

source: downloads/boost_1_34_1/libs/preprocessor/doc/topics/motivation.html @ 29

Last change on this file since 29 was 29, checked in by landauf, 16 years ago

updated boost from 1_33_1 to 1_34_1
File size: 5.3 KB
Line 
1<html>
2<head>
3        <title>motivation.html</title>
4        <link rel="stylesheet" type="text/css" href="../styles.css">
5</head>
6<body>
7<h4>Motivation</h4>
8<div>
9        The C++ function and template parameter lists are special syntactic constructs, and it is impossible to directly
10        manipulate or generate them using C++ constructs.&nbsp;
11        This leads to unnecessary code repetition.
12</div>
13<div>
14        Consider the implementation of the <code>is_function&lt;&gt;</code> metafunction is Boost.&nbsp;
15        The implementation uses an overloaded <code>is_function_tester()</code> function that is used for testing if a type is convertible
16        to a pointer to a function.&nbsp;
17        Because of the special treatment of parameter lists, it is not possible to directly match a function with an arbitrary parameter list.&nbsp;
18        Instead, the <code>is_function_tester()</code> must be overloaded for every distinct number of parameters that is to be supported.&nbsp;
19        For example:
20</div>
21<div class="code"><pre>
22template&lt;class R&gt;
23yes_type is_function_tester(R (*)());
24
25template&lt;class R, class A0&gt;
26yes_type is_function_tester(R (*)(A0));
27
28template&lt;class R, class A0, class A1&gt;
29yes_type is_function_tester(R (*)(A0, A1));
30
31template&lt;class R, class A0, class A1, class A2&gt;
32yes_type is_function_tester(R (*)(A0, A1, A2));
33
34// ...
35</pre></div>
36<div>
37        The need for this kind of repetition occurs particularly frequently while implementing generic components or metaprogramming facilities,
38        but the need also manifests itself in many far simpler situations.
39</div>
40<h4>Typical Solutions</h4>
41<div>
42        Typically the repetition is done manually.&nbsp;
43        Manual code repetition is highly unproductive, but sometimes more readable to the untrained eye.
44</div>
45<div>
46        Another solution is to write an external program for generating the repeated code or use some other extra linguistic means such as a smart editor.&nbsp;
47        Unfortunately, using external code generators has many disadvantages:
48        <ul>
49                <li>Writing the generator takes time.&nbsp; (This could be helped by using a standard generator.)</li>
50                <li>It is no longer productive to manipulate C++ code directly.</li>
51                <li>Invoking the generator may be difficult.</li>
52                <li>Automating the invocation of the generator can be difficult in certain environments.&nbsp; (Automatic invocation is desirable for active libraries.)</li>
53                <li>Porting and distributing the generator may be difficult or simply takes precious time.</li>
54        </ul>
55</div>
56<h4>What about the preprocessor?</h4>
57<div>
58        Because C++ comes with a preprocessor, one would assume that it would support these kinds of needs directly.&nbsp;
59        Using the preprocessor in this case is highly desirable because:
60        <ul>
61                <li>The preprocessor is highly portable.</li>
62                <li>The preprocessor is automatically invoked as part of the compilation process.</li>
63                <li>Preprocessor metacode can be directly embedded into the C++ source code.</li>
64                <li>Compilers generally allow viewing or outputting the preprocessed code, which can be used for debugging or to copy and paste the generated code.</li>
65        </ul>
66</div>
67<div>
68        Most unfortunately, the preprocessor is a very low level preprocessor that specifically does not support repetition or recursive macros.&nbsp;
69        Library support is needed!
70</div>
71<div>
72        <i>For detailed information on the capabilities and limitations of the preprocessor, please refer to the C++ standard <a href="../bibliography.html#std">[Std]</a>.</i>
73</div>
74<h4>The Motivation Example Revisited</h4>
75<div>
76        Using the primitives of the preprocessor library, the <code>is_function_tester()</code>'s could be implemented like this:
77</div>
78<div class="code"><pre>
79#include &lt;boost/preprocessor/arithmetic/inc.hpp&gt;
80#include &lt;boost/preprocessor/punctuation/comma_if.hpp&gt;
81#include &lt;boost/preprocessor/repetition.hpp&gt;
82
83#ifndef MAX_IS_FUNCTION_TESTER_PARAMS
84#define MAX_IS_FUNCTION_TESTER_PARAMS 15
85#endif
86
87#define IS_FUNCTION_TESTER(Z, N, _) \
88   template&lt;class R BOOST_PP_COMMA_IF(N) BOOST_PP_ENUM_PARAMS(N, class A)&gt; \
89   yes_type is_function_tester(R (*)(BOOST_PP_ENUM_PARAMS(N, A))); \
90   /**/
91
92BOOST_PP_REPEAT(BOOST_PP_INC(MAX_IS_FUNCTION_TESTER_PARAMS), IS_FUNCTION_TESTER, _)
93
94#undef IS_FUNCTION_TESTER
95</pre></div>
96<div>
97        In order to change the maximum number of function parameters supported, you now simply change the <code>MAX_IS_FUNCTION_TESTER_PARAMS</code> definition and recompile.
98</div>
99<hr size="1">
100<div style="margin-left: 0px;">
101        <i>© Copyright <a href="http://www.housemarque.com" target="_top">Housemarque Oy</a> 2002</i>
102</div>
103<div style="margin-left: 0px;">
104Permission to copy, use, modify, sell and distribute this document is granted provided this copyright notice appears in all copies.&nbsp;
105This document is provided "as is" without express or implied warranty and with no claim as to its suitability for any purpose.
106</div>
107        <hr size="1">
108        <div style="margin-left: 0px;">
109                <i>© Copyright <a href="http://www.housemarque.com" target="_top">Housemarque Oy</a> 2002</i>
110                </br><i>© Copyright Paul Mensonides 2002</i>
111        </div>
112        <div style="margin-left: 0px;">
113                <p><small>Distributed under the Boost Software License, Version 1.0. (See
114                accompanying file <a href="../../../../LICENSE_1_0.txt">LICENSE_1_0.txt</a> or
115                copy at <a href=
116                "http://www.boost.org/LICENSE_1_0.txt">www.boost.org/LICENSE_1_0.txt</a>)</small></p>
117        </div>
118</body>
119</html>
Note: See TracBrowser for help on using the repository browser.